1. Technical Field of the Invention
The present invention is concerned with a helical-ribbon mixer having a rotation-symmetrical inner mixing vessel wall, a motor-driven central agitator axis therein, on which, on lateral agitator arms, at least one helical ribbon is held close to the vessel wall, downwardly sloping with a circular slope in a direction of rotation.
2. Prior Art
A helical-ribbon mixer of this type is known from DE 195 47 220 C2. The helical ribbon serves to convey the material to be mixed in an upward flow near the wall, whereupon it flows back in a downflow near the axis and intermixes in the process. One problem associated with these mixers is emptying them as residue-free as possible, for which purpose the agitator arm that holds the helical ribbon at its lower end is designed with a special shape, so that it clears the vessel bottom while placing little stress on the material to be mixed. The helical ribbon is oriented radially relative to the central agitator axis and designed in a circular helical shape with the helix pitch preferably being relatively small near the bottom and increasingly larger from there, to allow for the initial acceleration of the material to be mixed. However, the slope of the helix surface relative to the horizontal is altogether less than the gliding angle so that the material to be mixed does not completely run off it during idling, so that additional measures must be taken to free the helix surface from the material to be mixed when a complete change of the mixing batch is to take place. Furthermore, the radially flat orientation of the helix surface has the shortcoming that it must be designed narrow relative to the mixer radius, as the screw helix, in the case of a cylindrical vessel chamber, will otherwise only rotate a hollow-cylindrical portion of the material in the mixer in a circular motion, with relatively stable separating borders forming in this rotating region and no up and down flows resulting. The essential precondition for the upward conveyance is the presence of a friction with a circular action whereby the material to be mixed is imparted with a lifting force component by the upwardly sloped helix surface, which, however, especially in the case of wider screw helixes, which are gentle on the material, and a relatively wide gap between the helix and the mixer wall, which also has a gentle effect, is only moderately developed.
Furthermore, a mixing apparatus is known from EP 1 123 731 A2, the housing of which conically widens from the bottom up at a cone angle of 60–120 degrees and subsequently transitions into a vertical cylinder space. A flat screw helix is located at the lower outlet and disposed adjoining same close to the wall are flat, strip-shaped mixer blades, which are elliptical on their outside, which may be pitched between 15 degrees and 60 degrees relative to the horizontal. These mixer blades produce a pronounced material displacement toward the center and a minor upward flow since the upper side of the blades is in each case sloped at an obtuse angle to the vessel wall and only a low pushing resistance is offered to the material to be mixed, which is required for a lifting force. Furthermore, no self-emptying takes place at the flat helix surface and with the flat mixer blade pitch. Also, at cone angles above 90° the housing is not self-emptying.